Vacuum processing operations take place in vacuum chambers that include chucks for supporting substrates in near vacuum or other low-pressure environments. Some such chucks merely provide a substrate support platform and rely on gravity to hold the substrates in place. Others actively secure the substrates with either mechanical or electrostatic clamps.
Some such chucks are also involved with the processing of the substrates by producing electrical or magnetic fields or by regulating heat transfers to or from the substrates. In plasma-assisted processes, electrical fields (e.g., chuck RF bias) direct or distribute plasma and related plasma ions impinging on the substrate. In data-storage device applications, magnetic fields can be used to magnetically orient films during their deposition onto substrates or during their subsequent thermal annealing. Heat transfers are used to remove excess heat produced by such processing operations or to provide a controlled amount of heating to assist the processing of the substrates. For example, some operations are best performed at constant substrate temperatures or at substrate temperatures that are adjusted throughout different stages of the operations.
During operations like thermal depositions (e.g., CVD) and annealing, elevated temperatures actually accomplish the substrate processing. For instance, heat-generating chucks for controlling substrate temperatures (e.g., up to 450° C.) are required for PVD reflow depositions of aluminum (Al) or copper (Cu) interconnect materials. Metal-organic chemical-vapor deposition (MOCVD) processes for depositing semiconductor interconnect materials (e.g., Al or Cu) or barrier materials (e.g., TiN or TaN) also require heat-generating chucks for controlling substrate temperatures (e.g., up to 350° C.).
However, controlling substrate temperatures in near vacuum or other low-pressure environments is quite difficult because heat does not transfer well at pressures approaching a vacuum. For example, the conduction of heat between contiguous surfaces of a chuck body and the substrate is slow and inefficient because actual contact on an atomic scale between the surfaces is limited to a small fraction of their common area, and gaps that separate the remaining areas of their surfaces are sufficient to prevent effective heat transfer by conduction.
Heating and cooling of substrates through radiational heat transfers are possible in a vacuum environment, particularly at elevated substrate and chuck temperatures; but radiational heat transfers are generally too slow to maintain substrates at desired processing temperatures. This is particularly true for most chuck-based fabrication processes with substrate temperatures below 450° C. Faster transfers are possible by pumping a gas, preferably an inert gas such as helium or argon or another gas such as nitrogen, between the chuck body and the substrate. Although still at much less than atmospheric pressure, the gas sufficiently fills the small gaps between the chuck body and the substrate to support significant heat transfer through thermal conduction between them. A seal formed between the mounting surface of the chuck body and the substrate resists significant leakage of the gas into the rest of processing chamber.
U.S. Pat. No. 4,680,061 to Lamont, Jr. discloses chucks having heating or cooling elements for regulating substrate temperatures. One of the chucks has a ceramic heating element mounted in a cavity between a chuck body and a substrate. The heating element is mounted close to a back side of the substrate but not in contact. Argon gas is introduced into the cavity to promote heat exchanges between the heating element and the substrate. A raised rim of the chuck body on which the substrate is mounted contacts a peripheral portion of the substrate's back side forming a seal that inhibits leakage of the gas out of the cavity.
Another of Lamont, Jr.'s chucks has a chuck body that functions as a heat sink with coolant channels for conveying heat from the sink. A similar cavity is formed by a raised rim in the chuck body so that the remaining heat sink is positioned close but not in contact with the back side of a substrate. Argon gas is similarly trapped within the cavity by contact between the raised rim of the chuck and the back side of the substrate.
U.S. Pat. No. 4,949,783 to Lakios et al. also discloses a chuck using gas pressure against a back side of a substrate to promote substrate cooling. A similar cavity is formed in the chuck body and surrounded by a raised rim for contacting the back side of the substrate. However, instead of merely pumping gas into the cavity, Lakios et al. circulate the gas both into and out of the cavity by establishing a gas flow. Part of the heat transfer from the substrate is due to gas-conducted heat exchanges with the chuck body, and another part of the heat transfer is due to the removal of heated gas from the cavity.
The chucks of both Lamont, Jr. and Lakios et al. include raised rims on their chuck bodies that function as both mounting surfaces and seals. Mechanical clamps press the substrates against the raised rims of their chuck bodies to tighten the seals and to reduce leakage of back side gas into their processing chambers. Lakios et al. also use an O-ring seal next to their raised rim to provide an even tighter seal for further reducing leakage.